Why standards matter for the machine you buy
The HVAC duct standard your project complies with is not just a paperwork question. It dictates the gauge of sheet metal you cut, the seam type you form, the reinforcement spacing, the leakage rate at acceptance test and the tolerances your machine must hit. If the standard is specified after you have bought the duct line, retrofitting compliance is significantly more expensive than configuring it correctly up front.
SBKJ engineers have configured auto duct lines for projects on all four major standards over the past three decades. This guide is the comparison we walk customers through when their project specification mentions a standard they are not familiar with — so they can specify the right machine, the right tooling and the right PLC recipe before the contract is signed.
SMACNA — North America, Middle East, international airports
SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) publishes the dominant ductwork construction standard used in the United States, Canada, the Gulf region and on most international airport projects. The two key documents are HVAC Duct Construction Standards — Metal and Flexible (rectangular) and HVAC Duct Construction Standards — Round Industrial (round and oval).
SMACNA defines pressure class in inches of water gauge (in.wg). Common classes are -3, -2, -1, +1, +2, +3, +4, +6 and +10 in.wg. Duct gauge, reinforcement type and joint type are tabulated against pressure class and the larger duct dimension — for example, a 600 mm wide duct at +2 in.wg requires 22-gauge galvanized steel with a TDF or angle flange joint and no intermediate reinforcement up to 1.2 m of unsupported length.
Sealing is graded as Class A, B, C or unsealed. Class A is the tightest (joints, seams and penetrations all sealed) and is required for plenums, supply duct above the ceiling and most medium-pressure systems. Class B seals joints and seams only. Class C seals only transverse joints. The leakage limits are documented in our SMACNA Seal Class A, B, C explainer.
Acceptance testing is done by the SMACNA Duct Air Leakage Test method — the duct is pressurised to its design class, leakage is measured with a calibrated orifice plate, and the result is compared to the maximum allowable rate for the seal class in CFM per 100 ft² of duct surface area at the test pressure.
EN 1505 / EN 1506 — Europe
EN 1505 is the European standard for sheet metal rectangular ductwork dimensions and tolerances. EN 1506 is the equivalent for circular ductwork. EN 1507 covers leakage testing for rectangular duct, and EN 12237 covers round duct leakage. Together these documents form the European duct construction framework, harmonised by CEN (European Committee for Standardisation) and adopted in EU member states plus Switzerland, Norway and the UK (alongside DW/144).
EN 1505 defines pressure class as A, B, C or D. Class A is up to 500 Pa, Class B up to 1000 Pa, Class C up to 2000 Pa and Class D up to 3000 Pa. Note that European pressure classes are stated in pascals not inches of water gauge — the conversion is 1 in.wg ≈ 249 Pa, so SMACNA +2 in.wg ≈ 500 Pa, roughly matching EN Class A.
Leakage classes are A, B, C and D, where A is loosest and D is tightest. The leakage limits in EN 1507 are stated in litres per second per square metre of duct surface area at the test pressure — a different unit and a different test methodology than SMACNA.
Reinforcement and gauge tables in EN 1505 are different from SMACNA. The gauge values are stated in millimetres (0.5, 0.6, 0.75, 0.88, 1.0, 1.2 mm) versus SMACNA's American Wire Gauge (24, 22, 20, 18 ga). Reinforcement spacing and type at a given pressure class often differ by 100–300 mm from SMACNA.
AS/NZS 4254 — Australia and New Zealand
AS/NZS 4254 is the joint Australian and New Zealand standard for ductwork construction. AS/NZS 4254.1 covers low-pressure ductwork and AS/NZS 4254.2 covers high-pressure ductwork above 500 Pa. The standard is mandated by the National Construction Code (NCC) in Australia and the Building Code in New Zealand, and is referenced in most large commercial and industrial HVAC project specifications.
AS/NZS 4254 borrows heavily from the European framework — pressure classes are stated in pascals, leakage testing methodology is similar to EN 1507, and gauge values are in millimetres. However, the reinforcement and tolerance tables are calibrated to local Australian construction practice, with specific clauses on seismic restraint and fire-rated penetrations that reflect Australian Building Code requirements.
The standard distinguishes between low-pressure (Class 1, up to 500 Pa) and medium/high-pressure (Class 2, up to 2500 Pa) systems with different gauge, joint and acceptance criteria. Most Australian commercial buildings are designed at Class 1; data centre and cleanroom projects routinely specify Class 2.
DW/144 — United Kingdom
DW/144 is the UK specification for sheet metal ductwork, published by BESA (the Building Engineering Services Association, formerly HVCA). DW/144 is the dominant fabrication specification on UK construction projects and is referenced in most UK Mechanical and Electrical specifications.
DW/144 broadly aligns with EN 1505 but adds UK-specific clauses on installation tolerance, joint sealing material specifications, fire-rated duct construction (DW/144 Section 19) and acceptance testing procedure. The pressure class definitions are aligned with EN 1505 (Low, Medium, High) but the acceptance testing protocol uses a UK-specific calibration.
For global fabricators bidding into UK projects, DW/144 compliance is normally a contractual requirement and the specification document is mandatory reading — many UK consultants specify DW/144 by reference and then audit the fabricator's quality control records against the document during the project.
GB standards (for international context)
For completeness, the national standard for HVAC duct fabrication is a combination of GB/T 17919 (sheet metal ductwork construction) and GB 50243 (HVAC installation acceptance code). These standards are aligned with EN 1505 in pressure class structure but with region-specific gauge and reinforcement tables. Most global export projects specify SMACNA or EN 1505 by buyer preference, with GB/T applied only to domestic GB-standard projects.
Side-by-side comparison table
The table below summarises the key differences across the four major standards. Use it as a quick reference; always consult the source document for binding clauses on a specific project.
- Pressure class units: SMACNA uses in.wg. EN 1505, AS/NZS 4254 and DW/144 use Pa.
- Gauge units: SMACNA uses American Wire Gauge (AWG). EN, AS/NZS and DW/144 use millimetre thickness.
- Joint types: SMACNA permits TDF, slip-on flange, drive cleat and angle flange. EN 1505 permits flanged joints, slip joints and welded joints. AS/NZS 4254 broadly aligns with EN 1505. DW/144 specifies flanged joints with detailed gasket material requirements.
- Sealing: SMACNA Class A ≈ EN Class C ≈ AS/NZS Class C ≈ DW/144 High. The exact equivalence is approximate; verify against source documents for project compliance.
- Reinforcement spacing: All four specify reinforcement intervals based on duct dimension and pressure class, but the actual spacing values differ by 100–300 mm at any given operating point.
- Leakage test method: SMACNA uses CFM/100 ft² at test pressure. EN, AS/NZS and DW/144 use l/s per m² of duct surface at test pressure. Conversion is non-linear because of the different reference units.
- Geographic scope: SMACNA in USA, Canada, GCC, international airports. EN 1505 in EU + Switzerland + Norway. AS/NZS 4254 in Australia + New Zealand. DW/144 in UK + Ireland.
How to specify the right standard for your project
If you are a building owner, consultant or contractor planning an HVAC project, the standard is normally inherited from the project specification — your M&E consultant or the project owner specifies it in the design package. Before procuring duct or duct-making machinery, confirm which standard is binding by reading the M&E specification.
If you are a fabricator setting up a duct production line, your standard is determined by the markets you serve. North American or Gulf market fabricators should configure their line for SMACNA. European fabricators for EN 1505. Australian and New Zealand fabricators for AS/NZS 4254. UK fabricators for DW/144. Multi-region exporters benefit from a multi-standard PLC, which costs about 5–8% more than a single-standard line but eliminates retooling between project standards.
How SBKJ machines comply
Every SBAL-V and SBAL-III auto duct production line shipped from SBKJ since 2018 includes multi-standard PLC recipes for SMACNA, EN 1505, AS/NZS 4254 and DW/144 as standard. The operator selects the project standard at job start; the PLC adjusts seam selection, reinforcement spacing, gauge thresholds and tolerance bands automatically. The first article from a multi-standard line should pass the project's acceptance test on day one.
SBKJ provides standard-specific test reports as part of FAT (Factory Acceptance Test). For SMACNA projects we run a SMACNA Class A leakage test at +2 in.wg. For EN 1505 we run an EN 1507 Class C test at 1000 Pa. For AS/NZS 4254 we run the AS/NZS 4254.2 test method. For DW/144 we run the BESA-prescribed High pressure test. Test certificates travel with the machine and are accepted by most international project consultants without further verification.
The flagship SBAL-V auto duct line and the SBTF series spiral tubeformers are configured for compliance with all four standards by default. The TDF flange machine and the SBLC lockformer support SMACNA TDF, EN 1505 flanged joint and AS/NZS 4254 flange profiles via tooling change-out. See SBAL-V product page and TDF flange machine for technical details.
When standards conflict — which wins?
Occasionally a project specification references multiple standards (for example, a Saudi airport project specifying SMACNA and a separate EU consultant adding an EN 1505 clause). The contract document hierarchy normally resolves this — the prime contract specification overrides any subsidiary documents — but the practical answer is to fabricate to the tighter of the two standards. The tighter spec passes both tests; the looser spec fails the tighter test.
For sealing class equivalence: SMACNA Class A is approximately equivalent to EN Class C, AS/NZS Class C and DW/144 High. If your contract specifies SMACNA Class A AND EN Class C, fabricate to EN Class C (which is functionally tighter at the leakage test point) and your duct will pass both inspections.
Standards revisions to watch
SMACNA is currently in a multi-year revision cycle for the rectangular duct construction standard, with a new edition expected in 2027. EN 1505 was last revised in 2008 and a CEN technical committee is reviewing material clauses. AS/NZS 4254.1 was revised in 2012 and AS/NZS 4254.2 in 2024. DW/144 was last revised in 2013. SBKJ engineering tracks each revision cycle and updates the PLC recipes within 60 days of any binding clause change.
For projects that span standard revisions (e.g. a 3-year build project in Australia spanning the AS/NZS 4254.2 2024 revision), confirm in the contract which version of the standard applies — the version current at contract signing, or the version current at first-article inspection. This is normally a project specification clause; if it is silent, default to the version current at first-article inspection.
Get an SBKJ quote configured for your project standard →
FAQ
What is the difference between SMACNA and EN 1505?
SMACNA is the North American standard with pressure classes in in.wg and seal classes A/B/C. EN 1505 is the European standard with pressure classes A/B/C/D in Pa and leakage classes A/B/C/D. They overlap functionally but are not interchangeable — gauge tables, reinforcement spacing and sealing requirements differ on every duct size.
Which standard applies in the Middle East?
SMACNA dominates the Gulf region (Saudi Arabia, UAE, Qatar, Kuwait, Bahrain, Oman) because most major projects are specified by US or UK consultants. EN 1505 appears occasionally on European-led projects.
Which standard applies in Australia and New Zealand?
AS/NZS 4254.1 (low pressure) and AS/NZS 4254.2 (high pressure) are mandated by the National Construction Code in Australia and the Building Code in New Zealand. SBKJ supplies SBAL-V and SBAL-III lines configured for AS/NZS 4254 compliance for Australian customers.
Can one auto duct line meet multiple standards?
Yes. The SBKJ SBAL-V auto duct line stores PLC recipes for SMACNA, EN 1505, AS/NZS 4254 and DW/144 as standard. The operator selects the standard at job start; the line adjusts seam type, reinforcement spacing and tolerance bands automatically.
What is DW/144 and where is it used?
DW/144 is the UK specification for sheet metal ductwork, published by BESA. It is used across the UK and parts of Ireland and on UK-led projects globally. DW/144 broadly aligns with EN 1505 with UK-specific clauses on installation tolerance and acceptance testing.
